During the production of printed wiring assemblies or printed circuit boards on which electronic components, such as transistors, capacitors, etc. are mounted, many contaminants are present thereon. For example, the side of the printed circuit board which is to be soldered is generally contaminated by solder fluxes and other resinous materials, and in addition, considerable residue can also lodge on the top or mounting surface of these circuit boards. Rosin flux residues left on printed circuit boards and other electronic assemblies after soldering contain activators and their decomposition products (amine hydrochlorides [RNH.sub.3.sup.+ Cl.sup.- ] being typical). These residues, which are ionic in nature, can lead to electrical leakage and cause corrosion if not completely removed after the soldering operation. While the rosin itself is non-ionic and non-corrosive, it too should be completely removed after soldering since it encapsulates ionic residues from flux activators. Unremoved flux residues can also lead to sticking and malfunctioning of component switches and can cause mealing and adhesion problems with conformal coatings. Complete removal of flux residues is, therefore, a key factor in guaranteeing long-term reliability of electronic circuits and components. Many methods have been described in the past for cleaning these printed circuit boards. These have included the scrubbing of the boards themselves, both on the bottom and top surface thereof, and the use of various solvent compositions in a liquid bath, including the use of stirring, ultrasonic waves, etc.
One method for cleaning printed circuit boards is disclosed in German Pat. No. 2,403,428, owned by Imperial Chemical Industries, Ltd. That patent teaches the use of certain specific solvent compositions in the cleaning of printed circuit boards in a bath or liquid system. That patent is thus concerned with a technique commonly referred to as "kiss cleaning", in which the cleaning liquid comes into contact only with the bottom side of the printed circuit board. In particular, the patentee teaches azeotropic compositions of 1,1,1-trichloroethane and aliphatic alcohols containing at least 2 carbon atoms. He also teaches such combinations of 1,1,1-trichloroethane and aliphatic alcohols containing at least 3 carbon atoms in order to raise the flash point of the mixture as the number of carbon atoms are increased. He therefore prefers to use alcohols such as butanol, pentanol, etc. The patentee does disclose the use of normal propanol in his application as well as the use of certain stabilizers in connection with the 1,1,1-trichloroethane composition. He does not, however, concern himself with suitable compositions for vapor cleaning processes, nor with mixtures that have the complete absense of a flash point.
In addition, an alternative process for cleaning printed circuit boards is disclosed in German Pat. No. 2,321,880 to Voldeholms A.B. This patentee discloses the use of a cleaning solution which contains an alcohol containing from 4 to 6 carbon atoms and a chlorinated carbon soluble solution such as 1,1,1-trichloroethane, perchloroethylene and trichlorethylene. These compositions are again disclosed as useful in various liquid cleaning systems, with the cleaner being used at temperatures between 20.degree. and 50.degree. C. with the aid of a brush. The patentee also notes that his invention is advantageous because the flash point of many of these compositions is not reached until the boiling point of the azeotrope is reached. Again he does not concern himself with suitable compositions for vapor cleaning processes, nor with mixtures that have the complete absence of a flash point, even at their boiling point.
Vapor cleaning or vapor degreasing processes utilize the cleaning mechanism whereby the vapor of boiling solvent is condensed directly on the assembly being cleaned, thereby dissolving and carrying away the soil. Such vapor cleaning processes are generally considered more desirable than cleaning solution, liquid bath or cold cleaning processes since in vapor cleaning processes evaporation losses are more readily controlled, the workpiece emerges from the process in an already dry condition, and the cleaning can be only in pure vapor and cleaner distillate, thereby avoiding contamination from workpieces already cleaned in the process. Vapor cleaning can be accomplished in batch type degreasers whereby soldered assemblies are accumulated in metal baskets and placed in the degreaser, or with in-line degreasers whereby soldered assemblies are conveyed through the degreaser. Besides cleaning in the solvent vapor, degreasers can be designed to allow cleaning by immersion in the boiling liquid, in the warm liquid distillate, or spraying with liquid distillate. Usually a combination sequence such as vapor-spray-distillate-vapor is employed.
In the past there have also been attempts to utilize various solvent compositions, particularly various azeotropic compositions, in such vapor degreasing procedures for cleaning printed circuit boards. Thus, in patents such as U.S. Pat. No. 3,671,442 and 3,671,446, and others, various such azeotropic compositions are disclosed. These compositions have the disadvantage of consisting largely of an expensive fluorinated solvent, thereby limiting their widespread use in electronic cleaning applications. Furthermore, some ot these compositions have a flash point or contain a C-4 or higher alcohol which is not particularly effective as an additive for removing ionic flux activators.
An ideal solvent for use in vapor degreasing processes and the like would, of course, not have a flash point, even at its boiling temperature. Secondly, the mixture should contain a sufficiently polar solvent in a concentration sufficient to enable the removal of ionic flux residues such as activators, as well as a non-polar solvent to remove non-polar soils like rosin and soldering oils. Additionally, it would also be most desirable to employ an azeotropic mixture or near azeotropic mixture so that the composition of the mixture remains relatively constant under normal operating conditions, and so that it may be recovered for reuse. Lastly, the mixture should not contain any expensive components that would limit it from being used in widespread printed circuit board and other related electronic equipment cleaning, including the cost sensitive home products electronics industry.